The invention relates generally to the field of ceramic materials and processes for making ceramic materials. More specifically, the invention relates to silicon carbide bodies having particles or inclusions which are dispersed within the bodies.
Silicon carbide is useful in a wide variety of applications due to its tribological characteristics and its outstanding thermal, chemical and mechanical properties. Such applications include, for example, mechanical seals, valve lifters, and other applications where a part is frictionally engaged with another material. For example, in many mechanical seal applications, the seal interfaces are subjected to both a large compressive stress normal to the seal surface and to high rotational speeds or sliding velocities. Such conditions are typically represented by the parameter PV which represents the product of the compressive stress and the sliding velocity.
When such a mechanical seal is used in a pump or agitator, the mechanical seal should provide adequate sealing of the working fluid. Conveniently, the working fluid may also serve to lubricate and cool the seal interface. If sufficient lubrication and cooling is not provided, excessive wear or catastrophic failure of the mechanical seal may result. For example, if insufficient fluid is provided at the seal interface during operation, the lubricant can vaporize due to the heat produced and cause catastrophic failure.
Hence, when a silicon carbide body is used in a seal or other bearing face which runs against the face of another material, the seal or bearing face should be exposed to a lubricating and cooling fluid (or used in fluid applications) so that a film may be produced between the sliding surfaces to lubricate and cool the surfaces, thereby reducing friction, wear, and temperature as well as catastrophic failure potential. Further, proper lubrication will tend to minimize power consumption.
To facilitate proper lubrication, a variety of silicon carbide materials have been proposed. These include both reaction bonded silicon carbide materials and sintered silicon carbide materials with special modifications to the standard product. The reaction bonded silicon carbide materials are produced by placing a carbon containing preform in contact with molten silicon metal. As examples of such processes, U.S. Pat. Nos. 4,536,449 and 4,120,731 describe a reaction bonded silicon carbide body having secondary lubricating particles dispersed therein. The complete disclosures of both these patents are herein incorporated by reference.
The above processes have met with limited success for a variety of reasons. For example, the processes used to produce such materials are often complex and can therefore be relatively expensive. For instance, as recognized in U.S. Pat. No. 5,656,563, the disclosure of which is herein incorporated by reference, it is difficult to incorporate large amounts of graphite into a ceramic matrix without causing cracks to occur in the microstructure or without increasing the material""s porosity. Producing a reaction bonded silicon carbide/graphite material is difficult because such a material typically has a residual silicon phase which limits corrosion resistance due to a reaction with the silicon in some chemical applications and also reacts with the included graphite to convert it to SiC. Further, controlling the reaction bonding process to obtain fully reacted and fully dense parts is difficult.
Hence, it would be desirable to provide silicon carbide materials having improved lubricity while maintaining the integrity of the microstructure. It would further be desirable to provide exemplary processes for making such materials. Such processes should be relatively simple so that the overall cost of the material may be reduced. Such a silicon carbide material should also be useful in applications having a high PV limit or temporary dry running applications while reducing the chances of catastrophic failure, excessive wear, and power consumption.
The invention provides exemplary silicon carbide ceramic bodies and processes for making such ceramic bodies. The silicon carbide ceramic bodies of the invention comprise silicon carbide in major amount and unreacted particles of an additive in minor amount. The particles are dispersed throughout the silicon carbide and provide a degree of lubricity when the ceramic body is operated against an operating surface. The additive comprises a material that is inert with respect to silicon so that the additive may remain intact during the manufacturing process. Exemplary additives which may be employed include: boron nitride, aluminum nitride, titanium diboride, and the like.
In one particular aspect, the ceramic body comprises from about one weight percent to about 25 weight percent of the additive. Preferably, the ceramic body comprises from about 12 weight percent to about 15 weight percent of boron nitride. In another aspect, the ceramic body comprises at least about 75 weight percent of silicon carbide.
The ceramic body may be fashioned into a variety of parts or components using diamond abrasive grinding wheels as the fired ceramic body is not machineable with single point tooling. For example, the ceramic body may comprise a pump seal, a bearing, a turbine component, a pump lifter, a nozzle, or the like.
The invention further provides an exemplary process for producing a reaction bonded silicon carbide body. According to the process, an additive which is inert with respect to silicon is incorporated with a carbon source and a silicon carbide source to form a raw batch mixture. The mixture is then exposed to molten silicon, siliconized, to produce a reaction bonded silicon carbide body having particles of the additive dispersed therein.
Prior to siliconization, an organic binder is preferably incorporated into the raw batch mixture to provide green strength and single point tooling machinability. The raw batch mixture is then compacted into a green body, which in turn, is exposed to a liquid silicon metal to produce the reaction bonded silicon carbide body.
Also prior to siliconization, an agent which is reactive with respect to the additive and the silicon carbide matrix is preferably incorporated into the raw batch mixture. This agent preferably comprises one or a mixture of the following: titanium, titanium dioxide, boron, boric acid, nickel, cobalt, manganese, or any other materials which is reactive with respect to the particular additive.
Such a process is particularly advantageous in that a lubricating material may rather easily be dispersed within the silicon carbide raw batch prior to compaction, thereby reducing the manufacturing cost. Moreover, because the additive does not react with the molten silicon metal, the integrity of the resulting microstructure is generally not compromised. Moreover, the active agent bonds with the additive and the silicon carbide matrix, thereby providing enhanced retention of the additive during operation conditions.
The carbon source for the reaction bonding process may comprise any one of a variety of carbon sources including graphite, carbon black, pyrolized resins, and the like. In one exemplary aspect, the silicon carbide powder is mixed into a slurry of the carbon source, the reactive agent, and the binder to form a secondary slurry. The secondary slurry is then dried, such as by using a spray dryer, to form a stock. The stock is then blended with the additive prior to compaction. The invention may employ the use of a wide variety of organic binders including polyvinyl alcohol, acrylic resin, coal tar pitch, long chain fatty materials, metallic stearates, sugars, starches, alginates, polystyrene, and the like.
In one particularly preferable embodiment, the raw batch mixture includes about 10 weight percent to about 30 weight percent carbon source, about 4 weight percent to about 8 weight percent reactive agent, about 88 weight percent to about 55 weight percent silicon carbide powder, about 1 weight percent to about 10 weight percent binder, and about 1 weight percent to about 25 weight percent additive. After compaction into the green body, the green body is preferably heated to a temperature in the range from about 1500xc2x0 C. to about 2000xc2x0 C. to facilitate exposure of the liquid silicon metal to the green body.
By using a raw batch mixture having about 10 wt % to about 30 wt % carbon source, and about 88 weight percent to about 55 (and more preferably about 60) weight percent silicon carbide powder, a more complete conversion of the carbon into silicon carbide is achieved. Because of the lack of residual graphite, a material is produced having a relatively high density and which is relatively hard, i.e. hard enough that is not machineable using single point tooling. For example, a silicon carbide body may be produced having an average density of at least about 2.80 g/cc, more preferably at least about 2.95 g/cc. The high density also contributes to the stiffness, hardness, flexural strength, thermal conductivity, abrasion resistance, and other physical characteristics that allow the material to be used as a mechanical seal, among other applications.
In another embodiment, the invention provides an exemplary raw batch mixture for producing a reaction bonded ceramic body. The raw batch comprises a carbon powder and a silicon powder in major amount. An additive in minor amount is also included. The additive preferably comprises boron nitride, aluminum nitride and/or titanium diboride. A reactive agent in minor amount is also included. The reactive agent preferably comprises titanium or titanium dioxide. In one aspect, a binder in minor amount is also provided. In one particularly preferable aspect, the additive comprises from about 1 weight percent to about 25 weight percent boron nitride and the reactive agent comprises from about 4 weight percent to about 8 weight percent.